Typically, semiconductor memory devices are classified into one of two categories, namely, volatile memories and nonvolatile memories. Volatile memory devices such as dynamic random access memory (DRAM) and static random access memory (SRAM:) are capable of retaining information only when supplied with power. In contrast, nonvolatile memory devices such as the read only memory (ROM) retain the data they store even when they are not supplied with power.
Currently, nonvolatile memory devices are classified as either a floating gate type or a metal insulator semiconductor (MIS) type. MIS type devices are formed so as to be double or triply layered by two kinds of dielectric layers.
The floating gate type of memory device is implemented with storage characteristic using potential wells. Flash electrically erasable programmable read only memory (EEPROM) is an example of the floating gate type of memory device. Flash EEPROM has an EPROM Tunnel Oxide (ETOX) cell structure and split gate type structure.
However, the ETOX cell has a drawback. In particular, the available cell size increases even though its simple layered structure is implanted in a small size since the drain contact should be formed along a bit line. That is, since the drain contact should be formed for each cell, a large area is required to secure the contact area and an overlap area between the contact and active region per cell.
The split gate cell has 2 transistors per cell, (i.e., a select transistor without a floating gate and a storage transistor with the floating gate that are serially connected with each other). However, the split gate cell also has drawbacks. In particular, the cell size increases by adding the select transistor to each cell, and the cell size further increases to secure the margin for self-aligning the channels of the select transistor and the storage transistor with the respective gates.
To clarify multiple layers and regions, the thickness of the layers are enlarged in the drawings. Wherever possible, the same reference numbers will be used throughout the drawing(s) and accompanying written description to refer to the same or like parts. As used in this patent, stating that any part (e.g., a layer, film, area, or plate) is in any way positioned on (e.g., positioned on, located on, disposed on, or formed on, etc.) another part, means that the referenced part is either in contact with the other part, or that the referenced part is above the other part with one or more intermediate part(s) located therebetween. Stating that any part is in contact with another part means that there is no intermediate part between the two parts.